1. Field of the Invention
The present invention generally relates to spectral analysis systems that can be used for polymer identification. More particularly, the invention particularly relates to improvements in Raman polymer identification systems to permit effective and rapid identification of darkly colored plastics.
2. Description of the Prior Detection Method
Many plastics that should be recycled, particularly in the automotive industry, are black or highly pigmented. Such darkly colored plastics have proven to be the most difficult to identify using existing plastic identification technologies. Due to the strong optical absorption of black plastics, most of the signal needed to perform a spectroscopic identification is absorbed by the sample and thus unavailable for detection. At the same time, absorption may also lead to a significant thermal change such as a rapid heating, melting and even burning of the plastic sample during the identification process. Thus, not only are the signal levels from black plastics very small, but also these weak signals, particularly Raman signals, may be further obscured by large interfering backgrounds due to the thermally induced changes in the plastics including smoking. For example, white plastics can be easily and rapidly identified in 0.1 seconds with a Raman spectrometer, such as that disclosed in International Publication WO 99/01750, using a 1 Watt diode laser power, while black plastics cannot be identified under the same conditions due to laser induced detrimental changes. The power density reaches 5 W/mm2when a 1 W laser at a wavelength of 800 nm is focused to a 0.5 mm diameter focal spot.
In order to avoid laser induced detrimental changes in the plastic, it is necessary to decrease the laser power density on the surface of the black plastic. One way to reduce laser power density is to reduce total laser power that illuminates the surface of the black plastic. But at same time, in order to accumulate enough signal for identification, the signal collection time has to be increased proportionally. Obviously, this is not acceptable for rapid identification. The other way to reduce the power density of the laser is to increase the size of the laser spot that illuminates the surface of the plastic, while still maintaining a sufficiently high laser power of 1 Watt to allow rapid identification. Experiments have shown that to avoid laser induced detrimental changes in black plastic samples, in the case of 1 Watt total laser power at wavelength 800 nm, the size of the laser spot illuminating the surface of a black plastic sample needs to be increased 40 times, to a size that is greater than 3 mm in diameter. As a consequence, the signal acceptance area of the collection fiber bundle and the acceptance area of the spectrograph (slit-height times slit-width) must also be increased 40 times. It is almost impossible to achieve this from a technical point of view. Enlarging the laser spot size without changing the optical train and components would cause the signal from the sample to overfill the collection fiber bundle and thus decrease the collected signal intensity.
Thus, there exists a need for a quick yet effective way to identify materials such as darkly colored plastics using spectral analysis, particularly Raman spectroscopy.
In order to aid in the understanding of the present invention, it can be stated in essentially summary form that it is directed to a moving objective lens in a traditional system employing spectral analysis such as a Raman polymer identification system. By moving the objective lens, the laser beam can be distributed to a focal plane, while the spectral signal from the moving laser spot can still be collected back to the same point as if the objective lens were stationary. As a result, the average power density of the moving laser spot can be reduced to a point that no light induced detrimental degradation such as undue heating, melting and burning of the plastic sample will occur, while still maintaining the same laser beam power level. At the same time the power level of the spectral signal being returned from the sample is maintained at a level sufficient to make very rapid identification of the character and composition of the sample by an analysis of the Raman or other spectral signal.
In a preferred system, an optical fiber bundle conducts the spectral signal from the sampling optics situated to receive the characteristic spectrum produced from the sample to a spectral analyzer. Even though the lens is moving, the spectral signal returns from the sample to the same point of the entrance end of the fiber bundle as if the lens were stationary. This means the spectral signal is not reduced by the movement of the lens. The terms moving and movement as employed in this application is to be given the broadest possible meaning and include a patterned or random movement of the lens so that the laser energy directed toward the sample is distributed over an area in the focal plane of the objective lens larger than would be achieved were the lens not subjected to movement. Examples of movement that are easily achieved to obtain the desired results include rotation of an eccentrically positioned objective lens, and one-or two-dimensional translational vibration of an objective lens in a plane roughly parallel to the sample surface.
The invention provides a convenient way to solve the problems faced by traditional Raman or other spectral polymer identification methodologies that prevent the rapid and effective identification of darkly colored plastics. The invention can be better understood from the following description when considered in conjunction with the accompanying drawings.